The present invention relates to a vehicle park brake status monitoring method.
Heavy duty vehicles such as trucks, buses, coaches as well as large trailers or semi-trailers are generally equipped with an air brake system whereby brakes are equipped with compressed air brake actuators on all axles.
A typical air brake system includes two air circuits, namely a service brake circuit and a park brake circuit.
The service brake circuit—which is usually divided into two independent circuits for safety reasons—is connected to a pedal upon which a vehicle driver can apply a foot brake pressure. The brake signal can be fed to an Electro pneumatic Brake System (EBS). The EBS electronically controls the service brake circuit. Depending on the pedal position, the EBS generates an electrical signal and redundantly a pneumatic signal. The electric signal is transmitted to a CPU. According to various parameters such as for example the vehicle's load situation, the vehicle speed and/or acceleration, the CPU calculates the appropriate pneumatic pressure for the vehicle brake cylinder to obtain a vehicle deceleration. The pneumatic signal is used in redundancy cases and performs similar functions.
FIG. 1 illustrates a typical brake cylinder 1 wherein an expandable diaphragm counteracting a return spring transfers the air pressure to a brake pushrod 5. The displacement of the brake pushrod activates a wheel braking mean for developing friction forces such as brake shoes of a drum brake or brake pads of a disc brake.
The park brake circuit operates under a different principle. The park brake circuit is usually connected to a hand lever. The hand lever controls the input of air pressure into a park brake chamber wherein a large power spring is counteracted by a flexible diaphragm which is connected to the park brake rod. When the park brake is not activated, air pressure is applied to the diaphragm; this counteracts the power spring and maintains the park brake rod in a first inward position. In contrast, when the park brake is actuated (or when there is a failure in the vehicle air system) air pressure is no longer applied to the diaphragm; the spring which is then not counteracted pushes the park brake rod and the vehicle brake is applied. The brake cylinder 1 depicted on FIG. 1 is typically fitted on a rear axle whereon park brake pressure is applied. As this is illustrated on FIG. 1, the park brake pushrod is in tandem with the brake pushrod. Thereby when the park brake is actuated, the park brake pushrod pushes the brake pushrod into a second outward position.
To ensure a satisfactory running of the vehicle, it is important that the vehicle driver is informed of the park brake status i.e. applied or released. To this end, the park brake status is usually based on the measure of the air pressure in the park brake chamber. A pressure sensor or a pressure switch can be fitted on the input port of the park brake chamber so as to monitor the pressure in said chamber.
When no air pressure is measured in the park brake chamber, the park brake is regarded as activated; a dashboard light and/or a reminder buzzer can notify the driver that the park brake is applied. In contrast when air pressure is measured in the park brake chamber, the park brake is regarded as released.
The current method of monitoring the park brake status is not entirely satisfactory insofar as this method only indirectly reflects the actual status of the park brake.
In the common occurrence of a park brake rod jammed by frost, the current method detects the park brake as released; in such a case, air pressure is duly detected in the park brake air chamber although the park brake rod is still jammed in an active position by frost or by any mechanical hitch. The driver is notified a message according to which the park brake is released                the dashboard light is turned off—and may drive on although the park brake is actually activated.        
It therefore appears that there is room for improvements in the way the park brake status of heavy vehicles is monitored.
It is desirable to provide a pneumatic park brake monitoring method capable of reflecting the actual park brake status. The method of the present invention provides a method for monitoring a status of a park brake of a vehicle brake system that includes a brake cylinder having a service brake system and a park brake system; the service brake system includes a service brake chamber that is delimited by a service brake displaceable wall; a brake pushrod is fixed upon the service brake displaceable; the brake pushrod is connected to a vehicle braking means and is capable of moving under a pneumatic pressure from a first position whereby the braking means are released to an a second position whereby the braking means are actuated; the park brake system having actuating means capable of pushing said brake pushrod into its second position; the method includes the steps of:                measuring the actual volume V of the service brake chamber;        deriving from said actual volume V an actual park brake status. The park brake status monitoring method according to this invention is based on measuring the volume of the service brake chamber as the volume of the service brake chamber is a parameter which is representative of the actual position of the brake push rod and thus is representative of the actual park brake status. The method of the invention makes it possible to give the vehicle driver a true status of the park brake when said driver needs to apply or release the vehicle park brake. By monitoring the volume of the service brake chamber, the method of the invention can inform the driver of the actual position of the brake rod unlike the methods of the prior art which are generally based on a control of the air pressure of the service brake chamber.        
The method according to the invention comprises the further step of the method comprises the further step of comparing said actual park brake status with a selected park brake status.
In practical terms, the step of deriving an actual park brake status can comprise the step of comparing the actual measured volume with at feast one predefined volume (Va, Vr) of the service brake chamber corresponding to a predefined park brake status.
The method can comprise further the step of verifying that the actual volume V of the service brake chamber is equal or greater to a predefined volume Va of the service brake chamber corresponding to the actuated park brake status.
To inform a vehicle driver of the actual park brake status, the method can comprise the further step of switching on a signal of park brake actuation if the actual actuated park brake status matches the selected actuated park brake status.
The method can comprise further the step of verifying that the actual volume V of the service brake chamber is lower or equal to a predefined volume Vr of the service brake chamber corresponding to the released park brake status. To inform the vehicle driver of a park brake malfunction, the method can comprise the further step of switching on a hazard signal of park brake failure, if the actual actuated park brake status does not match the selected actuated park brake status. This method is therefore able to detect a park brake malfunction whereby the park brake is applied by the vehicle driver but the park rod is still in its first position. This method is also able to detect a common malfunction of the park brake whereby the brake rod is jammed in a second position while the park brake rod is not actuated and while no pneumatic pressure is applied in the service brake chamber. Prior to initiating the step of measuring the volume of the service brake chamber, the method can comprise a step of checking that the vehicle is in a state whereby the vehicle service brake is not actuated or that the air supply is at a nominal level. This step is of importance as it is not advisable to monitor the park brake status when the vehicle service brake is actuated as it may interfere with the normal braking procedure nor it is necessary to initiate park brake status monitoring method when the vehicle air supply is defective or the vehicle air supply is in a built up pressure phase when the vehicle engine starts and the air compressor starts its air pressurizing action. In a possible embodiment of the method according to the invention, the park brake actuation means includes a park brake chamber delimited by a flexible park brake diaphragm upon which a park brake pushrod is fixed, the park brake pushrod being in tandem with the brake pushrod so that when the park brake is activated the park brake pushrod pushes the brake pushrod towards its second position. In a preferred embodiment of the invention, the measure of the actual volume of the service brake cylinder includes the steps of:                applying a pneumatic test signal Psb in the service brake chamber,        measuring the response pressure of the service brake chamber,        comparing the response pressure of the service brake chamber against at least at least one threshold pressure value.        
In concrete words, the volume of the service brake chamber is estimated in a measuring cycle where a test air pressure is applied in the service brake chamber. The pneumatic response of the service brake chamber is then measured and compared against two preset pressure values representative of the two positions of the diaphragm each corresponding to an extreme position of the brake rod and therefore representative of the volume of the service brake chamber. The estimation of the volume relies on the fact that the pneumatic response of the service brake chamber depends on the position of the service brake diaphragm within the brake cylinder.
The response pressure of the service brake chamber can be compared against a threshold high pressure value Ph and against a threshold low pressure value Pl.
The pneumatic test signal can be applied to the service cylinder chamber at a pressure comprised between 0.1 bar and 0.5 bar. The pneumatic test signal has to remain low so as not to interfere with the normal brake operations. The pneumatic test signal can be applied for a period of time T1 comprised, for example, between 5 ms and 20 ms. The period of time T1 depends upon a vehicle specific pneumatic layout and brake chamber size.
The pneumatic test signal is maintained in the service brake chamber for a period of time T2 comprised, for example, between 5 ms and 10 ms. In any cases, the period of time T2 should allow enough time to obtain a significant pressure measurement.
The pneumatic test signal is purged from the service brake chamber over a period of time T3 of at least 50 ms to make sure that there is no test residual pressure in the service brake chamber.
Advantageously, the vehicle brake system includes an Electro pneumatic Brake System EBS having a CPU with I/O interface, a data memory such as a EEPROM or a Flash-PROM memory capable of storing T1, T2, T3 and a program memory such as a ROM memory, as the EBS incorporates the resources which are necessary to implement the method according to this invention. To determine the values of Ph, Pl, T1, the method can comprise a self learning sub routine having the steps of:                when the vehicle park brake is released, applying incremental test pressure signals during incremental periods of time T1i, until the test pressure reaches a brake threshold pressure stored into the EBS memory for a final value T1f of T1i;         entering into the EBS memory, the test pressure value T1f and a value of Ph as a fraction of brake threshold pressure;        when the vehicle park brake is applied, applying test pressure signals during the period of time T1f and measure the pressure maximum value;        entering into the EBS memory, the value of Pl as a factor of the measured pressure.        
The value of Ph can be, for example, equal to 0.9 of brake threshold pressure and the value of Pl can be for example equal to 1.1 of pressure maximum value;
T3 can similarly be set in a self learning procedure whereby:                the vehicle park brake is applied;        air pressure is applied for the period of time T1f;         air pressure is released for a measured period of time T3 which is equal to the period of time necessary to vent to atmosphere the service brake chamber multiplied by a safety factor.        
These and other advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as a non-limiting example, an embodiment of a method according to the invention.